PowerPC PCI DMA issues (prefetch/coherency?)

[Scott Wood]

Chris Pringle wrote:
> The kernel version is 2.6.26. Firmware is custom on a custom board.
>> Cache coherency on PCI DMA requires that the memory be mapped with the M
>> attribute on this chip, but that should be happening based on 
>> detection of
>> the core.
>>   
> I'm not sure where to look to verify this?

Check asm/cputable.h for CPU_FTR_NEED_COHERENT.  Make sure that 
CONFIG_8260 is one of the #ifdefs that turns that on.  It looks like 
that was in place by 2.6.26 in arch/powerpc.  I'm not sure what to look 
for in arch/ppc.

>> Also make sure that you park the bus on PCI and raise its arbitration
>> priority, as done at the end of fixup_pci in 
>> arch/powerpc/boot/cuboot-pq2.c.
>>   
> Since this is a reasonably recent kernel,

Not really, there was a fair amount of 82xx work in the mid-2.6.20s. 
The addition of CPU_FTR_NEED_COHERENT to 82xx was somewhere in that time.

Can you try 2.6.30?

> I'd guess that both of these 
> things are correct. I've had a quick look in that file and there is code 
> in there raising arbitartion priority and parking the bus.

Just because the code is there doesn't mean you're using it -- are you 
using cuImage?  Are you using arch/ppc or arch/powerpc?

Typically this would be done by firmware; it's only in cuboot because 
u-boot wasn't doing it.

>> BTW, you may want to post to linuxppc-dev at lists.ozlabs.org for
>> powerpc-specific issues, especially this kind of hardware issue.
>>
>>   
> I've just posted there :-)

Doh, sorry. :-)

> Interestingly, I've just turned off cache snooping and the problem has 
> got much worse. This has surprised me as I thought that part of the job 
> done by pci_map_sg was to flush the CPU cache

It only flushes the cache on hardware that doesn't do coherent DMA.

-Scott